Hot pressing dense ceramic sheets for electronic substrates and for multilayer electronic substrates

Abstract

Dense ceramic sheets suitable for electronic substrates are prepared by hot pressing ceramic green sheets containing ceramic powder and organic binders which leave no undesirable residue upon pyrolysis in the absence of oxygen. Boron nitride sheets made of boron nitride powder in a similar binder are placed on each side of the ceramic green sheet to form a composite. After hot pressing the composite so as to remove the binder and densify the ceramic, the BN layers are removed.

What is claimed is: 
     
       1. A process for producing dense ceramic electronic substrates comprising the following steps: (a) preparing at least one ceramic green sheet comprising ceramic powder and organic binders which leave no undesirable residue upon pyrolysis in the absence of oxygen;   (b) placing boron nitride sheets comprising boron nitride (BN) powder and organic binders which leave no undesirable carbonaceous residue upon pyrolysis in the absence of oxygen, on each side of the ceramic green sheet prepared in step (a) to form a composite;   (c) hot pressing the composite prepared in step (b) under sufficient temperature and pressure to densify the ceramic; and   (d) removing the BN layers on each side of the dense ceramic sheet.   
     
     
       2. A process according to claim 1, wherein the BN layer is removed by washing or grit blasting. 
     
     
       3. A process according to claim 1, wherein the boron nitride sheets in step (b) are laminated on each side of the ceramic green sheet. 
     
     
       4. A process for making multiple dense ceramic sheets simultaneously according to claim 1, wherein the boron nitride (BN) sheets are placed adjacent to the ceramic green sheets to produce a stack of the form BN-ceramic-[BN-ceramic] x  -BN where x is a positive integer prior to step (c). 
     
     
       5. A process according to claim 1, where the ceramic green sheet in step (a) is made by: (i) mixing ceramic powder, polyethylene and oil; and compounding the mixture at an elevated temperature to facilitate melting of the polyethylene;   (ii) molding the compounded mixture in step (i) to form sheets by extrusion through a die; and   (iii) removing the oil by extraction with an organic solvent.   
     
     
       6. A process according to claim 6, where the ceramic green sheets are formed in step (ii) by pressing the compounded mixture in step (i) into flat sheets. 
     
     
       7. A process according to claim 1, where the boron nitride sheet of step (b) is made by: (i) mixing boron nitride powder, polyethylene, and oil; and compounding the mixture at an elevated temperature to facilitate melting of the polyethylene;   (ii) molding the compounded mixture in step (i) to form sheets by extrusion through a die; and   (iii) removing the oil by extraction with an organic solvent.   
     
     
       8. A process according to claim 1, where the ceramic powder is primarily AlN. 
     
     
       9. A process according to claim 1, where the ceramic powder is Al 2  O 3 . 
     
     
       10. A process for producing dense ceramic electronic substrates having dense conductive metal circuit pattern(s) on the surface(s) comprised of the following steps: (a) preparing ceramic green sheets comprising ceramic powder and organic binders which leave no undesirable residue upon pyrolysis in the absence of oxygen;   (b) depositing the desired metal circuit pattern(s) to the ceramic sheets with a suitable metal which is compatible with the ceramic;   (c) placing boron nitride green sheets on each side of the ceramic sheet having metal circuit pattern(s) in step (b);   (d) hot pressing the metallized sheets under sufficient temperature and pressure to densify both the ceramic and the metal powders; and   (e) removing the BN layers from the sides of the dense ceramic sheet.   
     
     
       11. A process according to claim 10, wherein the desired metal circuit pattern is applied by screen printing a metal paste. 
     
     
       12. A process for making multiple metallized dense ceramic layers simultaneously according to claim 10, where prior to step (d) the boron nitride sheets are placed adjacent to the metallized ceramic sheets to produce a stack of the form BN-metallized ceramic-[BN-metallized ceramic-] x  -BN where x is a positive integer. 
     
     
       13. A process according to claim 10, where the ceramic powder is primarily AlN and the metal is W. 
     
     
       14. A process according to claim 10, where the ceramic powder is Al 2  O 3  and the metal is W. 
     
     
       15. A process according to claim 10, wherein the ceramic green sheet in step (a) is made by: (i) mixing ceramic powder, polyethylene and oil; and compounding the mixture at an elevated temperature to facilitate melting of the polyethylene;   (ii) molding the compounded mixture in step (i) to form sheets by extrusion through a die; and   (iii) removing the oil by extraction with an organic solvent.   
     
     
       16. A process according to claim 10, wherein the boron nitride sheets in step (c) are laminated on each side of the ceramic green sheet. 
     
     
       17. Dense ceramic electronic substrates having dense conductive metal circuit pattern(s) on the surface(s) made by the process of claim 10 wherein the lateral shrinkage of the metallization and warpage of the substrate are effectively eliminated. 
     
     
       18. A process for producing multi-layer ceramic electronic substrates with internal interconnected metal circuits at the various layers comprising the following steps: (a) preparing ceramic green sheets comprising ceramic powder and organic binders which leave no undesirable residue upon pyrolysis in the absence of oxygen;   (b) producing metal-filled vias in the ceramic sheet(s) of step (a) by punching holes in the ceramic sheets and filling with metal powders;   (c) depositing the desired metal circuit patterns to the ceramic sheets of step (b);   (d) laminating the desired number of ceramic sheets of step (c) together in a manner which maintains registration of the metallized patterns and vias on the various layers to form a green multi-layer ceramic-metal composite;   (e) placing boron nitride sheets on each side of the composite of step (d);   (f) hot pressing the BN-composite-BN product of step (e) under sufficient temperature and pressure to densify both the ceramic and the metal powders; and   (g) removing the BN layers on each side of the dense ceramic sheet.   
     
     
       19. A process according to claim 18, wherein the desired metal circuit pattern is applied by screen printing a metal paste. 
     
     
       20. A process according to claim 18, wherein the BN layer is removed by washing or grit blasting. 
     
     
       21. A process according to claim 18, wherein the boron nitride sheets in step (e) are laminated on each side of the ceramic green sheet. 
     
     
       22. A process according to claim 18, wherein the ceramic green sheet in step (a) is made by: (i) mixing ceramic powder, polyethylene and oil; and compounding the mixture at an elevated temperature to facilitate melting of the polyethylene;   (ii) molding the compounded mixture in step (i) to form sheets by extrusion through a die; and   (iii) removing the oil by extraction with an organic solvent.   
     
     
       23. A process for producing a multiple of multi-layer ceramic substrates simultaneously according to claim 18, where prior to step (f) the boron nitride sheets are placed adjacent to the multi-layer ceramic-metal composites of step (d) to produce a stack of the form BN-composite-[BN-composite] x  -BN where x is a positive integer. 
     
     
       24. A process according to claim 18, where the ceramic powder is AlN and the metal is W. 
     
     
       25. A process according to claim 18, where the ceramic powder is Al 2  O 3  and the metal is W. 
     
     
       26. A process for producing multi-layer ceramic electronic substrates according to claim 18, where step (b) is replaced by the following: (i) preparing a metal powder containing sheet comprising metal powders and organic binders which leave no undesirable residue upon pyrolysis in the absence of oxygen; and   (ii) punching a hole(s) in the ceramic green sheets of step (a), followed by placing the metal powder containing sheet of step (i) against the ceramic green sheets and forcing the metal containing material into the hole(s) in the ceramic green sheets by a punching action.   
     
     
       27. A process for producing multi-layer ceramic electronic substrates according to claim 18, where surface cavities are produced in the substrates by: (i) punching hole(s) in the ceramic sheets of step (a) which will reside on the surface of the composite of step (d); and   (ii) filling the holes in step (i) with BN tape material of similar thickness prior to the lamination of the layers in step (d).   
     
     
       28. Multi-layer ceramic electronic substrates with internal interconnected metal circuits at the various layers made by the process of claim 18 wherein the lateral shrinkage of the metallization and warpage of the substrate are effectively elminated.